1. Field of the Invention
The present invention relates generally to the field of optical fiber cables and more particularly to a preterminated optical fiber cable having at least one drop cable extending therefrom.
2. Description of the Prior Art
With the advent of local area networks and the relative broadband capabilities of fiber optic links, it has become commonplace for new communication systems to include fiber optic capabilities. In the use of optical fibers, optical fiber cables are provided for physical protection of the fibers in view of the fragile nature of the glass optical fibers. A main, or trunk cable, having a predetermined number of optical fibers, is normally used between two termination points in a communication system. At a point where a connection must be made to local homes or buildings, a splice is made to one or more optical fibers in the trunk cable for connecting a separate smaller sized cable, or drop cable. The drop cable is a small flexible cable and is separately routed to the local home or building.
In those systems in which drop cables are required to be spliced to the trunk cable, the required splice must be made on site. This is a very difficult and time-consuming procedure which entails accessing the cable, carefully opening the outer sheath, radial strength yarns, buffer tubes, etc., for accessing the specified optical fibers, splicing the specified optical fibers to the drop cable optical fibers, and sealing the spliced point. Additionally, the technician making the splice may be faced with such adverse working conditions as extreme weather or inconvenient cable placement.
The splice, or branch point of the optical fiber cable must have a number of features First, it must protect the optical fibers and splices from moisture and mechanical damage and provide an adequately strong anchoring point for the drop cable. It also must insure that the electrical continuity of every metallic cable component is maintained and insure that the minimum optical fiber bend radius is not violated. The branch point should also be re-enterable for future optical fiber branching. Thus, to provide a cable with these features, a splice closure is used.
Conventional splice closures are normally comprised of metal or plastic and are utilized to enclose the splice thereby protecting the optical fibers and splices from environmental damage. The splice closures provide a strong anchoring point for the drop cables so that if tensile forces are applied to the drop cables, the splices will not be affected. In addition, the splice closure comprises a large cavity therein for providing space for making the splices and for providing slack buffer tube and optical fiber storage space.
Conventional splice closures are either the "butt-end-type" or the "through-put-type" splice closure. Alternatively, the splice closure may be a composite of the two types being adaptable for either splice method.
The butt-end-type splice closure is configured so that the trunk cable enters and exits the splice closure from the same end. In contrast, the through-put-type splice closures are configured so that the trunk cable enters the closure at one end and exits at the other.
In both types of splice closures, however, the manner in which the splice is made is substantially the same. Within the splice closure, the buffer tubes of the trunk cable are accessed and are coiled a number of times so that an adequate amount of optical fiber is available for splicing. A portion of the buffer tube(s) encompassing the specified optical fibers is removed (approximately 24 inches) so that the specified optical fibers may be cut and prepared for splicing with the corresponding drop cable optical fibers. Although not all of the 24 inches of optical fiber is required for the splice, the slack optical fiber is desired so that splices may be redone if necessary or if future optical fiber branching is desired.
In a similar manner, the drop cable (or cables), is prepared for splicing with the specified trunk cable optical fibers. After the splice is made using well-known splicing methods, the splice is held in a splice tray which is mounted within the splice closure cavity. The splice tray prevents a splice from moving about within the closure upon closure movement. The splice closure is assembled by placing the coiled buffer tubes and slack optical fibers within the cavity and enclosing the cavity so that the splice is completely encapsulated.
Because the conventional splice closure must house the slack buffer tubes and optical fibers without violating the minimum bend radius of the optical fibers, the splice closure cavity must be somewhat large to provide adequate space for storage and splicing purposes. Ultimately, this results in a splice closure which is large and bulky, thus making it impractical for all other uses except for on-site optical fiber splicing.
For example, in those systems in which the cable route architecture has been determined, preterminated cables utilizing copper conductors or the like have been designed and utilized. A preterminated cable comprises a main trunk cable and several drop cables spliced to the trunk cable at various specified points, or branch points during the initial manufacture of the cable. The drop cables, which are usually less than 100 feet in length, may then be connected to the specified termination points upon cable installation. This facilitates the installation of cable thereby minimizing the time and cost. The drop cables may also be preconnectorized, or be assembled with the appropriate connectors at the time of manufacture, for easier and faster installation.
But, in the past, preterminated optical fiber cables have not been manufactured or utilized because of the simple reason that conventional optical fiber splice closures are large and bulky thereby making them impractical. Because the optical fiber trunk cable may be several thousand feet in length, it is normally wound in reels upon manufacture so that it may be stored for subsequent deployment. The conventional splice closure makes a preterminated optical fiber cable quite impractical and unwieldy to be installed in this manner. In addition, an optical fiber cable utilizing conventional splice closures may not be placed on small cable reels for deployment without incurring cable or fiber damage.
Furthermore, in many applications, it is desirable that the optical fiber cable be disposed within polyvinyl chloride (PVC) piping for additional environmental and rodent protection or pulled through such piping to conveniently pass under roads, driveways, etc. Because standard PVC piping has a four-inch inner diameter, it is impossible to install a preterminated optical fiber cable utilizing the conventional splice closure. In such a situation, the only manner of providing drop cable access to the trunk cable using the conventional splice closure is by making the splice after the trunk cable is deployed. Thus, providing a preterminated optical fiber cable having drop cables spliced thereto while maintaining optical fiber protection and minimal overall cable size is a difficult problem which had not been previously resolved.
Even with the current level of understanding of the conflicting needs associated with optical fiber cables, there has not previously been a practical preterminated optical fiber cable having multiple optical taps at a single branch point although such a cable is desirable.